1. Field of the Invention
The invention relates to a drive device for a light-emitting display panel in which, for example, organic EL (electroluminescent) elements are used as light-emitting elements and the elements are arranged, for example, like a plane, and, more particularly, to a drive device for a passive-drive-type light-emitting display panel in which the life time of the light-emitting elements can be extended by applying a forward voltage and a reverse direction voltage to the above-described organic EL elements in a predetermined period.
2. Description of the Related Art
Development of a display using a display panel in which light-emitting elements are configured to be arranged in a matrix has been widely promoted, and organic EL elements using an organic material for a light-emitting layer have been noticed as light-emitting elements used for such a display panel. The background is that the elements which are adequate for practical use and have higher efficiency and longer life time have been realized by using an organic compound, which can be expected to have good light-emitting characteristics, for the light-emitting layer of the element.
The above-described organic EL element can be electrically expressed by an equivalent circuit shown in FIG. 1. That is, the organic EL elements can be replaced by a configuration comprising a light-emitting element E, which consists of a diode element, and a parasitic capacitance compound Cp, which is connected to the light-emitting element in parallel. Accordingly, the organic EL elements are considered to be a capacity-type light-emitting element.
When a light-emitting control voltage is applied to the organic EL element, charges equivalent to the electric capacity of the element concerned, in the first place, flows into an electrode as a displacement current and are accumulated. Successively, when a predetermined voltage (light-emitting threshold voltage=Vth) unique for the element is exceeded, a current begins to flow from an electrode (the side of an anode of the diode element E) to an organic layer forming the light-emitting layer and light is emitted with an intensity in proportion to the current, according to a thought.
FIG. 2A to FIG. 2C show static characteristics of such organic EL element. According to the characteristics, the organic EL element emits light with a luminance (L) in proportion approximately to a driving current (I) as shown in FIG. 2A, and the current (I) rapidly flows to emit light as shown in FIG. 2B when a driving voltage (V) is larger than the light-emitting threshold voltage (Vth). In other words, the current hardly flows in the EL element and light is not emitted when the driving voltage is equal to or smaller than the light-emitting threshold voltage (Vth). Therefore, in a region in which the driving current is larger than the light-emitting threshold voltage (Vth) and light can be emitted as shown by a solid line in FIG. 2C, the luminance characteristic of the EL element has a characteristic in which the light-emitting luminance (L) is increased as the value of the voltage (V) applied to the element becomes larger.
A passive-drive-type display panel in which organic EL elements are arranged in a matrix has already been put into practical use by some companies as a display panel using such organic EL element. FIG. 3 shows one example of the passive-drive-type display panel and a drive device therefor. There are two methods for driving organic EL elements according to the passive-drive-type driving method: cathode-line scanning and anode-line drive; and anode-line scanning and cathode-line drive. The example shown in FIG. 3 is one form according to the former method of the cathode-line scanning and the anode-line drive.
That is, a display panel 1 has a configuration in which anode-lines Al to An as n pieces of data lines are arranged in the vertical direction; cathode lines K1 to Km as m pieces of scanning lines are arranged in the horizontal direction; and organic EL elements E11 to Enm, represented with symbol marks of a diode, as light-emitting elements are arranged at each intersecting place (n×m points in total).
And, in each EL element E11 to Enm forming a pixel, one end (the anode terminal of an equivalent diode to the EL element) is connected to the anode line, and the other one (the cathode terminal of the equivalent diode to the EL element) is connected to the cathode line, according to points of intersections between the anode-lines A1 to An in the vertical direction and the cathode lines K1 to Km in the horizontal direction. Moreover, the anode-lines A1 to An are connected to an anode drive circuit 2, and the cathode lines K1 to Km are connected to a cathode scanning circuit 3 for each driving.
The above-described anode-line drive circuit 2 has a configuration in which constant current sources I1 to In which is driven by a power supply voltage VH to generate a predetermined driving current and drive switches SX1 to SXn are comprised, and driving currents from the above-described constant current sources I1 to In are supplied to the anode-lines A1 to An through the drive switches SX1 to SXn, respectively.
That is, currents from the constant current sources I1 to In, respectively, are configured to be supplied to the EL elements E11 to Enm arranged corresponding to the cathode lines by selection of the side of the above-described constant current sources I1 to In with the above-described drive switches SX1 to SXn. Moreover, the anode lines are configured to be able to be connected by the above-described drive switches SX1 to SXn to the side of the ground as the reference potential point when the currents from the constant current sources I1 to In are not supplied to the individual EL elements.
On the other hand, the above-described cathode-line scanning circuit 3 has a configuration in which scanning switches SY1 to SYm are comprised corresponding to the cathode lines K1 to Km, respectively, and, for example, either of a reverse bias voltage VM obtained by dividing the above-described power supply voltage VH or the ground potential as a scanning reference point is connected to the corresponding cathode lines.
Thereby, light-emitting of each of the above-described EL elements is configured to selectively be performed by connecting the constant current sources I1 to In to the desired anode lines A1 to An, respectively, while the cathode lines are set at the scanning reference point (ground potential) with a predetermined cycle. Here, it is also possible to use a constant voltage circuit, instead of the above-described constant current source, as a driving power supply. But it is general to use the constant current source as the driving power supply, as shown in FIG. 3, because the current-luminance characteristic is stable to temperature changes, and, on the other hand, the voltage-luminance characteristic of the EL elements is unstable to the temperature changes.
Moreover, the above-described anode-line drive circuit 2 and the above-described cathode-line scanning circuit 3 are configured to display an image corresponding to image data according to the image data supplied to a light-emitting control circuit 3 by receiving instructions from the light-emitting control circuit 3 which is not shown in FIG. 3. In this case, the scanning switches SY1 to SYm are controlled in the cathode-line scanning circuit 3 to sequentially be switched so that any one of cathode lines corresponding to a horizontal scanning period of the image data is sequentially selected by an instruction from the light-emitting control circuit and is set at the ground potential as the scanning reference point and the reverse bias voltage VM is applied to other cathode lines in an non-scanning state. Here, FIG. 3 shows a state in which a second cathode line K2 is scanned and the reverse bias voltage VM is applied to other cathode lines.
There is a configuration in which the above-described reverse bias voltage VM charges the parasitic capacitance of the EL elements, which are connected to intersections with the cathode lines which have been selected for scanning, in a driving state and prevents the EL elements, which are connected to intersections between the anode lines in a driving state and the cathode lines which have been selected for scanning, from crosstalk light-emitting by leakage currents. Generally, the voltage value of the reverse bias voltage VM is set at an approximately equal value to a forward voltage vf of an EL element in a light-emitting state. Then, since the scanning switches SY1 to SYm are sequentially switched to the side of the ground potential for each horizontal scanning period, the EL elements, which have been connected to the cathode lines, can be put into a state, in which light-emitting can be performed, by the cathode lines set at the ground potential.
On the other hand, there is supplied a drive control signal from the above-described light-emitting control circuit to the above-described anode-line drive circuit 2, wherein light-emitting of any one of the EL elements connected to anode lines is performed and timing and duration of light-emitting of the EL element are controlled by the drive control signal, based on pixel information demonstrated by the image data. The above-described anode-line drive circuit 2 has a configuration in which on control of some of the drive switches SX1 to SXn to the side of the above described constant current sources I1 to In is performed in response to the drive control signal and driving currents are supplied to the EL elements through the anode lines A1 to An according to the pixel data.
Thus, light-emitting control of the EL elements to which the driving currents are supplied is performed according to the above-described image data. Here, since FIG. 3 shows the state in which the second cathode line K2 is scanned as described above and the drive switches SX2 and SX3 among the drive switches SX1 to SXn are connected to the side of the constant current sources, light-emitting control of the EL elements of E22 and E32, which are enclosed with a circle in the drawing, is performed.
Incidentally, it has been empirically known that the light-emitting life of the above-described organic EL elements can be extended by supplying the forward voltage according to the polarity of the equivalent diode and by applying the reverse direction voltage (reverse bias voltage), which does not contribute to sequential light-emitting operation, to the elements. The above point is disclosed, for example, in the Japanese Patent Application Laid-Open No. HEI-11 (1999)-8064 (paragraphs 0003 to 0005 and FIG. 2A to FIG. 2C).
Then, an operation during which anode lines which are excluded for lighting driving are connected to the side of the ground potential with the drive switches SX1 to SXn is performed in the configuration shown in FIG. 3. By the operation, the above-described reverse bias voltage can be applied to the EL elements arranged at the positions of intersections between the anode lines which are excluded for lighting driving and the cathode lines in an non-scanning state.
For example, in the state in which the second cathode line K2 is scanned for lighting driving of the elements E22 and E32 as shown in FIG. 3, the reverse bias voltage VM is applied to the cathodes through the scanning switch SY3, respectively and the anodes are connected to the side of the ground potential through the drive switches SX1 and SXn, respectively, when the elements E13 and En3 are noticed. Thereby, the reverse bias voltage of the voltage value VM is applied to the above-described elements E13 and En3, respectively. The operation during which the reverse bias voltage is applied is similarly performed when one-frame scanning is completed and the subsequent frame is scanned.
On the other hand, a case in which light-emitting of EL elements connected to specific cathode lines is not performed for a while may be generated for some pieces of data. Assuming, for example, a case in which light-emitting of EL elements connected to the third cathode line K3 is not performed for a while, the voltage of the cathode line K3 becomes the ground potential by scanning the third cathode line K3 and, at the same time, the voltages of EL elements connected to the cathode line K3 also become the ground potential. At this time, the voltages of the anodes of the EL elements connected to the above-described cathode line K3 become the ground potential through the above-described drive switches SX1 to SXn.
Accordingly, both the voltages at the anodes and those at the cathodes for the EL elements connected to the cathode line K3 become the ground potential together, there is no application of the forward voltage to the EL elements. Moreover, the above state continues over a considerable number of frames for some image data. Therefore, there is generated a problem that an effect by which the light-emitting life of the elements is extended becomes lower in a state in which there is no application of the forward voltage to the elements even if the reverse bias voltage is applied to the EL elements as described above.